Implantable cardioversion/defibrillation devices detect cardiac arrhythmias and deliver electrical pulses to the heart to treat the arrhythmia. The type of electrical stimulation therapy delivered depends on the type of arrhythmia detected. For example, ventricular fibrillation (VF) is a life-threatening condition and is treated immediately by delivering a high-energy defibrillation shock. Ventricular tachycardia (VT), though not immediately life threatening, is a serious condition typically treated first with anti-tachycardia pacing therapies and, if not successful, progressing to more aggressive therapies including high-energy cardioversion shocks if necessary. Atrial arrhythmias, which are sometimes conducted to the ventricles and are then referred to as “supraventricular tachycardias” or “SVTs,” are treated by delivering an appropriate anti-tachycardia pacing therapy or cardioversion/defibrillation shock to the atria.
Cardioversion/defibrillation shocks consume large amounts of battery energy and are painful to the patient. One challenge in the design of ICDs is accurately classifying a detected arrhythmia to thereby avoid unnecessary electrical shocks in the ventricle in response to SVTs. Studies have shown that SVTs may occur in up to 30% of ICD patients. In theory, the shape of the QRS complex in the EGM signal during SVT will not change significantly from the QRS complex during normal sinus rhythm (NSR) because ventricular depolarizations are caused by normal HIS-Purkinje conduction from the atrium to the ventricle. If high ventricular rates are due to a ventricular tachycardia (VT), one can expect a very different morphology of the electrogram (EGM) signal of the ventricular depolarization (QRS complex) because of a different pattern of electrical activity of the heart during VT.
One approach to accurately classifying arrhythmias includes examining the morphology of the QRS complex to discriminate normally conducted ventricular beats from abnormal ones based on the similarity of the EGM signal to a sample waveform recorded from the normal heartbeat. A reference template may be generated from a digitized sample waveform and comparisons made between a QRS complex during an unknown rhythm to a QRS template generated during a known rhythm, such as during NSR.
A number of patents describe the use of morphology analysis or template matching in arrhythmia detection and classification. Reference is made, for example, to U.S. Pat. No. 3,978,856 issued to Michel, U.S. Pat. No. 4,552,154 issued to Hartlaub, U.S. Pat. No. 5,273,049 issued to Steinhaus et al., U.S. Pat. No. 5,857,977 issued to Caswell et al., U.S. Pat. No. 5,447,519 issued to Peterson, and U.S. Pat. No. 5,718,242 issued to McClure et al.
One approach for morphology analysis is Correlation Waveform Analysis (CWA) or its less computationally costly counterpart, so-called Area of Difference Analysis (AD). Both require minimization of a function describing differences between two signals (sum of squared differences of wave data points for the case of CWA, and the sum of absolute values of the differences for AD). However such computations as typically performed are more computationally costly and require more power than is generally desirable within implantable ICDs. In U.S. Pat. No. 6,393,316 issued to Gillberg et al., incorporated herein by reference in its entirety, a method and apparatus for reliable discrimination between ventricular depolarizations resulting from normal and abnormal propagation of depolarization wavefronts by means of a wavelet transform based method of depolarization morphology analysis are generally disclosed.
One limitation that is encountered when comparing the morphology of a QRS waveform during an unknown rhythm to a NSR reference template is that the digitized QRS morphology may be altered due to a high ventricular rate associated with an SVT even though a true ventricular arrhythmia is not present. Therefore, in some instances, a fast ventricular rhythm due to an SVT may be incorrectly classified as VT or VF as the result of a mismatch between the depolarization waveform morphology during the fast ventricular rate and a NSR morphology template. It would be desirable therefore to provide a template of a ventricular depolarization waveform during a fast rate due to an SVT to allow morphology comparisons to be made during an unknown rhythm to an SVT morphology template. However, it is challenging to obtain an SVT morphology template in that the acquisition of the template must occur during a known SVT episode and inducing such episodes may be undesirable or impractical. EGM data storage during arrhythmia episodes allows a physician to identify SVT episodes from which a template might be generated. However, sorting through stored arrhythmia episode data to select an episode that best represents the EGM morphology during an SVT can become an arduous task for a physician.